Question: 2 notebooks cost $3.14. Which equation would help determine the cost of 13 notebooks?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 13 notebooks. Since 13 notebooks cost $x$ , we have the following proportion: $\dfrac{13}{x}$ We can write the fact that 2 notebooks cost $3.14 as a proportion: $\dfrac{2}{\$3.14}$ The cost changes along with the number of notebooks purchased, and so the two proportions are equivalent. $\dfrac{13}{x} = \dfrac{2}{\$3.14}$